A motor with a non-uniform air gap is described in the referenced and incorporated application Ser. No. 060,879 of July 26, 1979, refiled as a continuation-in-part under Ser. No. 220,181, filed Dec. 23, 1980, by the inventor hereof. Motors with non-uniform air gaps and which are pulse-energized are now known in the Art and have been described in both patent as well as technical literature, see, for example, German Pat. No. 23 46 380, assigned to the assignee of this application; and an article by the inventor hereof entitled "Two-Pulse Brushless d-c Motors - Construction and Operation", published in "asr-digest fur angewandte Antriebstechnik," issue 1-2/1977 (asr-digest for applied motive power technology), pages 27-31.
Two-pulse brushless d-c motors may be defined as motors which have a stator winding which receives two current pulses for each rotation of the permanent magnet rotor over 360 electrical degrees (.degree.-el). Typically, the pulses do not last for 180.degree.-el but are shorter. Current flow through the windings or coils of the stator, typically, is controlled by solid-state switching elements, such as transistors, which are controlled to conduction or cut-off, respectively, by a galvano magnetic sensor, for example a Hall generator.
Motors, for example as described in the aforementioned German Pat. No. 23 46 380, are suitable for many applications. Some applications require an extreme of uniformity of torque throughout the entire range of rotation of the rotor. The stator structure, using salient poles, will have gaps between the poles. These gaps are needed in order to permit winding of the armature windings on the stator poles. The presence of these stator pole gaps can become troublesome in selected applications where an extreme of uniformity of torque is required. If the motor or its connected elements have sufficient inertia, then the torque variations due to the stator pole gaps may not be material. The momentary or instantaneous variations in speed as the torque changes when a magnetically active portion of the permanent magnet rotor passes over the pole gaps are difficult to measure with the type of measuring equipment usually used to test motors. In applications in which the resolution of instantaneous speed is critical, however, for example in video recording equipment, momentary variations in operating speed may produce distortions in the resulting picture. These distortions, apparently, are caused by the rotor pole gaps passing over the stator pole gaps and receiving, first, a driving reluctance torque--due to the increase in air gap, and then a braking or retarding reluctance torque, due to the decrease in effective magnetic air gap.